1. Field of the Invention
This invention relates generally to novel forms of elemental silver and, more particularly, this invention relates to microporous elemental silver, methods of preparing the same and methods of preparing articles therefrom.
2. Description of Related Art
Electrochemical cells utilizing bipolar electrode designs having reactive metal electrodes supported on a substrate current collector are well-known. See, for example, Momyer et al, U.S. Pat. No. 4,269,907 (May 26, 1981), the disclosure of which is hereby incorporated by reference, wherein cells including an aqueous electrolyte, an anode of an alkali metal, such as lithium, for example, a cathode spaced from the anode, and an intercell electrical connector are disclosed. The intercell electrical connector typically comprises a substrate of a conductive metal (which may be in the form of foil or a plate) such as nickel or silver-plated nickel. The anode is formed on one surface of the substrate with the cathode of an adjacent cell on the opposite surface of the substrate. In such cells, the cathode may comprise an electrochemically active material, such as silver oxide, and the electrolyte may comprise an aqueous alkaline solution.
Momyer et al. also disclose an electrochemical cell stack comprising a plurality of bipolar electrodes connected in series.
Silver oxide electrodes are generally useful in silver oxide/lithium electrochemical cells as well as in other electrochemical power generating systems, such as silver oxide/aluminum, silver oxide/zinc, silver oxide/iron and silver oxide/cadmium cells. In the past, the silver oxide electrodes used by the silver battery industry have generally been fabricated either from chemically produced silver oxide powder or from metallurgically produced silver powder which in turn is oxidized to form silver oxide.
For example, in forming a silver oxide/lithium bipolar electrode according to the conventional process known as parallel oxidation, silver powder is first extruded onto a transfer paper from a rolling mill. This "biscuit" of silver is then sintered and hot forged onto a conductive metal foil substrate. The metal foil is generally made thin both for design considerations, e.g., weight and volume minimization, and economic considerations, i.e., cost minimization. For example, silver clad nickel foil substrates with a thickness of only about 1 mil are commonly used in the formation of silver oxide/lithium bipolar electrodes. The hot forgings of silver on metal substrates are then assembled in a stack in which the forgings of silver and nickel counter electrodes are alternated and separated by a non-conductive separator material to reduce the likelihood of short circuits developing between the substrates and the forgings of silver during the charging process.
All of the silver forgings in the charging stack are electrically connected in parallel for attachment to the positive terminal of a DC power supply. All of the nickel counter electrodes are in turn electrically connected in parallel for attachment to the negative terminal of the aforementioned DC power supply. The charging stack is then placed in an aqueous electrolyte, such as a metal hydroxide solution. Ionic current flow is generated through the aqueous electrolyte and the silver is thereby oxidized. The electrochemical process occurring at the nickel counter electrodes is cathodization which results in the release of hydrogen gas, as follows: EQU 4 H.sub.2 O+4e.sup.- .fwdarw.2H.sub.2 +4OH.sup.- (1)
The silver is electrochemically oxidized first to the monovalent state (see equation 2, below) and may then be oxidized to the divalent level or the peroxide state (see equation 3, below): EQU 2 Ag+2 OH.sup.- .fwdarw.Ag.sub.2 O+H.sub.2 O+2e.sup.- (2) EQU Ag.sub.2 O+2 OH.sup.- .fwdarw.2AgO+H.sub.2 O+2e.sup.- (3)
The theoretical electromotive force (EMF) for the oxidation of Ag to the monovalent state is +0.342 volt. The standard redox potential of oxygen is +0.401 volt and, consequently, oxygen gas does not evolve at the voltage level associated with the oxidation of Ag to the monovalent state. The second stage of oxidation of the active silver material, however, occurs at an electric potential of about +0.599 volt which is nearly 0.2 volt greater than the oxygen evolution voltage. Consequently, the oxidation process at this upper voltage level is inefficient as oxygen gas is evolved thereby.
Many applications require batteries having both a high capacity and a high rate of discharge. Further, because of design considerations, many applications require batteries having essentially flat electrodes. Flatness is a particularly important consideration in bipolar electrode configurations wherein both the anode and the cathode active materials are bonded on opposite sides of a conductive metal substrate.
The above-described parallel oxidation method of electrode formation, however, frequently results in bent electrodes. For example, the silver oxide electrodes made by the above-described method of oxidation are frequently of a bent, irregular shape. The bending of the electrodes is believed to be largely a result of the stoichiometric and molar volume changes which occur upon oxidation during electrode formation and is commonly referred to as "potato chipping".